Free cholesterol-rich and cholesterol ester-rich particles accumulate in atherosclerotic lesions. A lipid-rich fraction containing particles similar to those observed microscopically in histological preparations (see report Z01 HL 02823-03 EA) has been isolated from human atherosclerotic lesions. Chemical and morphological characterization of fractionated cholesterol-rich particles is being carried out and indicates that four subpopulations of particles exist, which vary in size and free and esterified cholesterol content. Results of these studies should help determine the origin of accumulated particles and possiblly suggest means to mobilize these pathological deposits of cholesterol.